iio: adc: axi-adc: make sure AXI clock is enabled

We can only access the IP core registers if the bus clock is enabled. As
such we need to get and enable it and not rely on anyone else to do it.

Note this clock is a very fundamental one that is typically enabled
pretty early during boot. Independently of that, we should really rely on
it to be enabled.

Fixes: ef040706

 ("iio: adc: adi-axi-adc: add support for AXI ADC IP core")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240426-ad9467-new-features-v2-4-6361fc3ba1cc@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>